The Canonical Wnt Signal Restricts the Glycogen Synthase Kinase 3/Fbw7-Dependent Ubiquitination and Degradation of Eya1 Phosphatase

Abstract

Haploinsufficiency of Eya1 causes the branchio-oto-renal (BOR) syndrome, and abnormally high levels of Eya1 are linked to breast cancer progression and poor prognosis. Therefore, regulation of Eya1 activity is key to its tissue-specific functions and oncogenic activities. Here, we show that Eya1 is posttranslationally modified by ubiquitin and that its ubiquitination level is self-limited to prevent premature degradation. Eya1 has an evolutionarily conserved CDC4 phosphodegron (CPD) signal, a target site of glycogen synthase kinase 3 (GSK3) kinase and Fbw7 ubiquitin ligase, which is required for Eya1 ubiquitination. Genetic deletion of Fbw7 and pharmacological inhibition of GSK3 significantly decrease Eya1 ubiquitination. Conversely, activation of the phosphatidylinositol 3-kinase (PI3K)/Akt and the canonical Wnt signal suppresses Eya1 ubiquitination. Compound Eya1^+/−; Wnt9b^+/− mutants exhibit an increased penetrance of renal defect, indicating that they function in the same genetic pathway in vivo. Together, these findings reveal that the canonical Wnt and PI3K/Akt signal pathways restrain the GSK3/Fbw7-dependent Eya1 ubiquitination, and they further suggest that dysregulation of this novel axis contributes to tumorigenesis.

ACKNOWLEDGMENTS

We are grateful to Bert Vogelstein for the HCT116 and HCT116 FBW7^−/− cell lines. We thank Xi He for providing Gsk3β and DKK1 expression plasmids and L-Wnt3a cells, Martin E. Dorf and Daniel Finley for the HA/UB expression plasmid, and Satoshi Kaneko for the IP kinase assay. We thank Ian Teng for technical support.

This work was supported by grants to X.L. from NIH/NIDCR (1R01DE019823), NIH/NIDDK (R01DK916451 and P50DK65298), and AHA (13GRNT16950006).